One example of such a keratoscope is to be found in the U.S. Pat. No. 4,772,115 to Martin Gersten, Richard J. Mammone, and Joseph Zelvin. In that patent a translucent conical illuminated ring device has a cylindrical bore or passage from its base to its tip lined with set of alternate transparent and opaque rings along the inside surface of the passage. Light from a light box at the base of the cone floods the interior of the cone's material, thereby "back-lighting" the transparent rings. When a curved reflective surface, e.g. the cornea of a patient's eye, is positioned at one end of the passage, an observer, or properly focused camera, looking into the bore's opposite end can acquire an image of the ring pattern appearing on, i.e., reflected from, the object. Variations in the radius of the rings in the pattern from circularity represent distortions of the curved surface.
It has been found to be desirable to be able to use illuminated ring devices having different diameter rings to accommodate the variation in human physiology and to produce different patterns of illumination that may appropriate for examining patients with different eye maladies. However, the computer controlled machine described in the above-mentioned patent is calibrated for use with a particular illuminated ring device having a predetermined diameter and pattern of rings. Substitution of a different device would require time-consuming re-calibration or re-programming of the machine for each different arrangement of light ring device employed.